I remember this very nice feature in Dungeon Siege 2: The mobs are shown as red dots on the minimap and also on the map. Also the NPC’s are shown as green dots. It makes it easier to make sure the player doesn’t miss cleaning a room or an area and it also helps discovering secret/hidden areas, because it stimulates the player to find an entrance to the area where they see red dots.
Having secret areas is a very good idea, but it’s unrealistic to expect the player to pay attention to every single feature in the walls and floors, all over the game. Instead, it’s better to give them some hints about a secret area, stimulating the player to find the entrances. In my view, of course.